How do i know if someone served in marine corp?

my fiancee says he was a marine sniper special ops. He has marine stickers and front license plate on his truck. he may have been dishonorably discharged. He asked me not to tell his 15 yr old son because he doesnt want his kid to know he killed people. Why advertise that on ur vehicle and expect a 15 yr old not to notice? Maybe stickers came on the truck lol. But this is serious. He proposed to me 3 days after we met on april 1st. asked me to quit my job in a bar (and i did), then broke up with me memorial day and took all the cash in my home,my birthday presents from him, my prescription meds, my engagement ring and other jewelry he had bought for me. Anyhow, his erratic behavior and pressure to immediately get married have worried me. All i need to know is if he is lying about the military service. He claims he was sniper, special ops and that there would be no records of his service.